This invention relates to an adaptive filter for filtering a received signal into a filtered signal in accordance with controllable tap gains.
A conventional adaptive filter has first through N-th taps and is supplied with a received signal through a transmission path to filter the received signal into a filtered signal in accordance with first through N-th controllable tap gains, where N represents a positive integer which is not less than one. The adaptive filter may be one of an echo canceller, a howling canceller, and an adaptive equalizer.
The received signal may be received through an input path at the adaptive filter. When taking a delay time of the input path into consideration, it is necessary to make the positive integer N be very large in order to filter the received signal into the filtered signal as desired. Therefore, it is practically difficult to filter the received signal into the filtered signal because the conventional adaptive filter uses the first through the N-th controllable tap gains.
In order to practically filter the received signal into the filtered signal, an improved adaptive filter is disclosed in "A Fast Algorithm for Adaptive FIR Filters with Adaptive Tap Location Based on Transition-Timing Control of Tap-Index Subgroups" which is published in Spring Conference of The Institute of Electronics, Information and Communication Engineers in 1993. The improved adaptive filter comprises first through N-th taps and a filter section for filtering the received signal into the filtered signal in accordance with first through K-th controllable tap gains, where K represents a positive integer which is less than the positive integer N.
The improved adaptive filter further comprises a tap selecting section for selecting selected ones of the first through the N-th taps as first through K-th selected taps in accordance with a tap select signal to connect to the first through K-th selected taps to the filtering section and a producing section for producing the tap select signal in accordance with the first through the K-th controllable tap gains.
However, it is difficult to filter the received signal into the filter signal at a rapid speed in the improved adaptive filter as will later be described. Namely, it is difficult to remove a residual echo in a rapid speed.